The power transmission apparatus for vehicle is provided in a power transmitting path from an engine to driving wheels to transmit power generated at the engine to each of drive wheels. Normally, it is constructed by a transmission, propeller shaft, final reduction device, drive shaft and the like. For example, “New-type Car Manual TOYOTA CELSIOR” (issued by TOYOTA JIDOSHA KABUSHIKI KAISHA in November, 1988) disclosed a general final reduction apparatus which includes a reduction device and a differential device constructed integrally. A driving torque of the engine is inputted into the power transmitting device and then is transmitted to the driving wheels, while a reacting torque balancing with the driving torque is inputted from the driving wheels into the power transmitting apparatus.
There is a demand for improvement of power performance of the vehicle, and an output of the power source, grip force of tire, suspension device and the like are improved to satisfy such the demand. Here, when the vehicle is started quickly, or the vehicle is running on a rough-surface road, the drive wheel(s) floated up momentarily from the road, and the driving torque and the reacting torque momentarily decreases.
Thenafter, at the moment of landing of the driving wheels on the road, extremely large driving torque and reacting torque are inputted into the power transmission apparatus. This may cause a temporary shortage of the transmission capacity of a rotary shaft such as a driving shaft, propeller shaft and the like, or capacity of gears arranged in a differential device, automatic transmission device and the like. Thus, durability of the shafts or gears may be lowered.
For overcoming the above problem, it is conceivable tor example to make size of the shafts and the gears large to thereby increase strength of them, whereby the power transmission capacity of the shafts and the gears is increased. However, there is fear that strengthened shafts and the gears may make the power transmission apparatus large-sized and heavy-weighed, so that quality of the vehicle may be lowered.